In digital communication systems, many voice channels are carried in a single piece of wire (or rather using a pair of wires, one for transmitting and the other for receiving).
As we all know the humans can hear sounds from 20Hz to 20kHz. When we speak, we put out a combination of frequencies varying (roughly) from 20Hz to 20kHz. For distance communication, we need to transmit this information over wires. If we try to transmit this voice signal as it is, it will eat up the bandwidth of the physical path available for communication. To convey the information needed and to identify the speaker, frequencies up to 4kHz is enough. According to Nyquist criteria the minimum frequency we need to sample this signal is 8kHz. If we use 8bits to represent the sampled signal, the bit rate will be 8000x8 bps (bits per second). i.e. 64kbps. This is the bit rate used in basic digital communication lines (in PSTN).
This 64kbps slot is called an E0. A combination of 32 of these slots is called an E1. That is an E1 is made out of 32 time slots, each carrying 8bits and the time used to transmit the whole E1 frame is 125us.
This gives a basic idea of an E1.
